Surely you've seen fossils in museums, but what about this: opalized
dinosaur tooth. Opalized fossils occur when silica settled into cracks
in the dinosaur bone and then hardened into opal. This one above is a
particularly fantastic specimen: an opalized
theropod dinosaur tooth from the Australian Museum.

I grind rough opals to make my own cabs and beads. What isn't mentioned here, is that the silica of an opal is usually embedded in some sort of rock matrix, like ironstone, and that silica compared to the matrix is soft and fragile. The dinosaur the tooth belonged to was a ferocious meat-eater. The material that replaced that tooth over time is beautiful, but easily damaged. Good thing it's in a museum where it can be protected.